The present invention relates to a method and apparatus for protecting a low voltage differential signal (hereinafter referred to as LVDS) driver when a power-off condition occurs. More specifically, the present invention provides for a method and apparatus that prevents a local LVDS port from being incorrectly initialized, and thereby prevents the receipt of false signals in a remote LVDS port from the local LVDS port.
Low Voltage Differential Signaling (hereinafter referred to as LVDS) is a technology used in data transmission systems. A low voltage differential signal produced by a line driver typically has peak-to-peak amplitudes in the range from 250 mV to 450 mV. The low voltage swing minimizes power dissipation, while maintaining high transmission speeds. Typical transmission speeds are over 100 Mbps (Mega-bits per second).
Electronic devices (i.e. computers) often have a local port that is a connection terminal for a remote port. When a remote port is connected to the local port, the local port is initialized to recognize the remote port. After initialization, the remote port and the local port can exchange information. The communication between the ports is often done by means of LVDS driver and receiver pairs.
Portable electronic devices such as cell telephones, personal data assistants (PDAs), and portable computers are commonly used in modern society. Since power is in a limited supply in such portable electronic devices, it is often necessary to periodically shut down the power used by various resources until it is needed. For example, hard disk drives are often powered down in portable computers until needed in an effort to reduce overall power consumption.
In accordance with the invention, an apparatus and method for protecting an LVDS line-driver when power is deactivated solves the above and other problems. A power-off protection circuit for an LVDS line-driver eliminates initialization problems in a local LVDS driver circuit that are caused by a remote LVDS driver when the local LVDS driver is disabled. The remote LVDS driver may introduce a signal into the substrate of the local LVDS driver when the local LVDS driver is in a power-off mode. A current source in the local LVDS driver couples power from a local power supply node to the local LVDS driver when power is active. A method and protection circuit connects the substrate of the current source to the local power supply when power is active, and decouples the substrate from the local power supply when power is deactivated. The remote LVDS driver cannot cause a false power supply signal in the local LVDS driver since the conduction path is disconnected. A first switching element couples a floating substrate node in the current source to the local power supply when the power is active. A second switching element couples the floating substrate node to a bias line when power is deactivated. The first switching element is deactivated by a rising potential in the floating substrate when the local power supply is in a power-off mode. The rising potential is caused by a signal that is transmitted by the remote LVDS driver. By isolating the floating substrate from the local power supply, false signals are eliminated and initialization problems are avoided.
In accordance with an aspect of the invention, a line-driver with a power-down protector that is active when power is disconnected from the line-driver includes a switching circuit that disconnects a conduction path in the line-driver when active, and a detector that determines when the line-driver is in a power-down mode and activates the switching circuit wherein the conduction path in the line-driver is disconnected from power such that false signals are disabled from transmission by the line-driver. The line-driver may be a low voltage differential signaling driver (LVDS driver).
In accordance with a further aspect of the invention, the line-driver includes a first transistor that is biased as a current source in a power-on mode and disabled in the power-down mode such that the first transistor provides the conduction path from power to the line-driver when power is connected to the line-driver. When the first transistor is a field-effect transistor, the gate terminal of the first transistor is coupled to the bulk terminal of the first transistor in the power-down mode. The gate terminal of the first transistor is coupled to a bias potential, and the bulk and source terminals of the first transistor are coupled together in the power-on mode. Also, the switching circuit may include a second transistor that is arranged to couple the gate terminal and the bulk terminal of the first transistor together when active. The switching circuit may also include a third transistor that is arranged to couple the bulk terminal of the first transistor to a local power supply when active. In another embodiment of the invention, the detector circuit includes the switching circuit.
In accordance with yet a further aspect of the invention, the detector circuit is arranged to enable the switching circuit in response to an output signal that approaches a local power supply terminal while power is disconnected from the local power supply terminal. In one embodiment of the invention, the detector circuit includes a first transistor that is arranged to couple the local power supply terminal to a floating substrate node when active, and a second transistor that is arranged to activate the first transistor when the local power supply is coupled to power. In another embodiment of the invention, the detector circuit includes a first transistor that is arranged to decouple a floating substrate node from the local power supply node when active, and a second transistor that is arranged to activate the first transistor when the local power supply is decoupled from power. The line-driver may include a second transistor that is a field effect transistor that is biased as a current source in a power-on mode and disabled in the power-down mode such that the second transistor provides the conduction path from power to the line driver when power is connected to the line-driver, and the floating substrate node is coupled to the bulk terminal of the second transistor. A third transistor may be arranged to activate as the local power supply node discharges towards a circuit ground when the power is disabled, and also arranged to deactivate the first transistor when the output signal increases towards the local power supply node.
In accordance with another aspect of the invention, a method provides for disabling a line-driver from transmitting false signals after a main power supply is disabled. The method provides for coupling a floating substrate node to a local power supply node such that the line-driver is free to transmit signals when the main power supply is active, and detecting a power-off condition where the local power supply node is decoupled from the main power supply. By decoupling the floating substrate node from the local power supply node when the power-off condition is detected, and coupling the floating substrate to a bias signal when the power-off condition is detected, the conduction path through the substrate is disabled and the line-driver is disabled from transmitting false signals. The method may also include coupling an output signal from the line-driver to the floating substrate node when the local power supply node is decoupled from the main power supply.
In accordance with yet another aspect of the invention, an apparatus prevents initialization problems in a local line-driver due to substrate conduction that is caused by a signal from a remote line-driver. The apparatus includes a first switching means couples a floating substrate node in the local line-driver to a local power supply node when active. A detector means detects a power-off operating mode for the local line-driver. A first activation means activates the first switching means when the operating mode is the power-on mode for the local line-driver. A second switching means couples the floating substrate node in the local line-driver to a bias signal node in the local line-driver when active. A second activation means activates the second switching means when the operating mode is the power-off mode such that substrate conduction is eliminated from the floating substrate node to the local power supply node in the local line-driver when operating in the power-off mode. Also, a deactivation means may deactivate the first switching means when local line-driver is operating in a power-on mode. The deactivation means may be arranged to deactivate the first switching means when an output signal node of the local line-driver is driven to a high potential by the remote line-driver. An isolation means is arranged to isolate the floating substrate node in the local line-driver from an output signal node of the local line-driver when the local line-driver is in the power-on operating mode. The first switching means may include the second switching means.
A more complete appreciation of the present invention and its improvements can be obtained by reference to the accompanying drawings, which are briefly summarized below, to the following detail description of presently preferred embodiments of the invention, and to the appended claims.